The present invention relates to fiber optic communication systems, and more particularly to a method and apparatus for recovering amplitude modulated vestigial-sideband ("AM-VSB") television signals from an optical fiber.
Cable television systems currently distribute television program signals via coaxial cable, typically arranged in tree and branch networks. Coaxial cable distribution systems require a large number of high bandwidth electrical amplifiers. For example, 40 or so amplifiers may be required between the cable system headend and an individual subscriber's home.
The use of a television signal comprising amplitude modulated vestigial-sideband video subcarriers is preferred in the distribution of cable television signals due to the compatibility of that format with NTSC television standards and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of AM-VSB transmission, however, is that it requires a much higher carrier-to-noise ratio (CNR) than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of AM-VSB television signals.
The replacement of coaxial cable with optical fiber transmission lines in television distribution systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems.
One problem in implementing an optical fiber distribution system, particularly for AM-VSB signals, is that semiconductor optical amplifiers of the type typically used in fiber optic systems produce high levels of distortion products that are not compatible with multi-channel AM-VSB video signals. This is due to the short lifetime of the carrier excited state within the semiconductor optical amplifier. The recombination time of such an amplifier operating near 1.3 .mu.m or 1.5 .mu.m is about 1.2 nanoseconds, which is short compared to the period of a typical AM-VSB subcarrier operating in the cable television band of about 55.25 MHz-1 GHz.
The dependence of second order distortion on carrier lifetime in a semiconductor optical amplifier is discussed in A.A.M. Saleh, et al., "Nonlinear Distortion Due to Optical Amplifiers in subcarrier-Multiplexed Lightwave Communications Systems", Electronics Letters. Vol. 25, No. 1, pp. 79-80, 1989. As noted in that article, second order nonlinear distortion is a significant problem in proposed lightwave cable television home distribution systems, where the use of semiconductor amplifiers to overcome inevitable distribution losses can potentially degrade system performance appreciably.
The difficulties presented in transmitting multi-channel AM-VSB television signals over fiber optic distribution systems have led others to propose the use of frequency modulation ("FM") instead of the more desirable AM-VSB format. See, e.g., R. Olshansky, et al., "Microwave-Multiplexed Wideband Lightwave Systems Using Optical Amplifiers for Subscriber Distribution", Electronics Letters, Vol. 24, No. 15, pp. 922-923, 1988; R. Olshansky, et al., "Subcarrier Multiplexed Passive Optical Network for Low-Cost Video Distribution", presented at OFC 1989; and W. I. Way, et al., "Carrier-to-Noise Ratio Performance of a Ninety-Channel FM Video Optical System Employing Subcarrier Multiplexing and Two Cascaded Traveling-Wave Laser Amplifiers", presented at OFC 1989. Another proposal has been to convert AM-VSB signals to a digital format for transmission. Digital transmission of AM-VSB television signals over an optical communication link is described in U.S. Pat. No. 4,183,054 to Patisaul, et al., entitled "Digital, Frequency-Translated, Plural-Channel, Vestigial Sideband Television Communication System".
An apparatus and method for transmitting AM-VSB television signals over a fiber optic distribution system in analog form is disclosed in commonly assigned co-pending U.S. Pat. application Ser. No. 07/454,772 entitled "Transmission of AM-VSB Video Signals Over an Optical Fiber", and incorporated herein by reference. That application discloses the modulation of light from a light source, such as a laser, with a signal having an AM-VSB subcarrier. The modulated light is passed through an optical amplifier having a long excited state lifetime with respect to the period of the subcarrier. The amplified modulated light output from the amplifier is coupled to an optical fiber which provides distribution of the signal to, for example, cable television subscribers. In a preferred embodiment, the optical amplifier is a doped fiber amplifier such as an Erbium-doped fiber amplifier.
When a plurality of AM-VSB signals are transmitted over an optical fiber, there must be some means for recovering each of the individual signals (e.g., television channels) at each subscriber location coupled to the distribution network. One way to accomplish this in the electrical domain is to convert the optical signals received at the subscriber location to electrical signals, and input the electrical signals to a conventional receiver to tune, demodulate, and reproduce a desired channel. There has heretofore been no way to efficiently recover individual AM-VSB channel signals in the optical domain instead of in the electrical domain.
The use of a semiconductor optical amplifier as an optical mixer has been suggested for the recovery of angle modulated subcarrier information, i.e., frequency or phase modulation. See, T.E. Darci, et al, "Optical Mixer-Preamplifier for Lightwave Subcarrier Systems", Electronic Letters, 1988, Vol. 24, pp. 179-180. There is no suggestion of using such a scheme for conversion of AM subcarrier modulation. The use of semiconductor optical amplifiers in an AM system has not been previously considered due to the incompatibility of such amplifiers with AM signal distribution as noted above. In particular, a semiconductor optical amplifier adds intermodulation distortion products when amplifying amplitude modulated subcarriers. Such distortion has been previously believed to render such amplifiers unusable for AM applications.
It would be advantageous to recover an individual AM-VSB channel in the optical domain, and then convert the recovered signal to the electrical domain for final processing. Such a technique would allow optical fiber distribution all the way to a subscriber's home. It would also simplify the necessary subscriber apparatus and provide increased reliability at a potentially lower cost.
It would be further advantageous to provide a method and apparatus for using a semiconductor optical amplifier as a mixer for the conversion of AM subcarrier modulation. Any such method and apparatus would have to overcome the effects of nonlinear distortion introduced by the amplifier. The amplifier could then be used as an optical mixer to allow recovery of individual AM channel signals in the optical domain. The present invention provides such a method and apparatus.